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Once prepare is completed, and all the various steps
that entails are done, the volume is ready to get “activated”.

This activation process enables a systemd unit that persists the OSD ID and its
UUID (also called fsid in Ceph CLI tools), so that at boot time it can
understand what OSD is enabled and needs to be mounted.

Note

The execution of this call is fully idempotent, and there is no
side-effects when running multiple times

The OSD uuid is being required as an extra step to ensure that the
right OSD is being activated. It is entirely possible that a previous OSD with
the same id exists and would end up activating the incorrect one.

The systemd unit will look for the matching OSD device, and by looking at its
LVM tags will proceed to:

# mount the device in the corresponding location (by convention this is

/var/lib/ceph/osd/<clustername>-<osdid>/)

# ensure that all required devices are ready for that OSD. In the case of
a journal (when --filestore is selected) the device will be queried (with
blkid for partitions, and lvm for logical volumes) to ensure that the
correct device is being linked. The symbolic link will always be re-done to
ensure that the correct device is linked.

# start the ceph-osd@0 systemd unit

Note

The system infers the objectstore type (filestore or bluestore) by
inspecting the LVM tags applied to the OSD devices

For exsiting OSDs that have been deployed with different tooling, the only way
to port them over to the new mechanism is to prepare them again (losing data).
See Existing OSDs for details on how to proceed.